The present invention relates generally to high frequency inverters and is specifically directed toward a fault protection system for a thyristor/capacitor inverter half-bridge.
There are many applications requiring the conversion of conventional 60 Hz line power into a high frequency signal for energizing an output device. One application is in the field of gaseous discharge lamps, such as fluorescent lamps, and high-intensity discharge (HID) lamps. These devices operate much more efficiently at frequencies on the order of 15-20 KHz, or higher, rather than the power input line frequencies of 60 or 120 Hz.
Despite the economic incentive to energize gas discharge lamps with a higher frequency, and even though many solid state ballast circuits have been proposed in the literature and are otherwise known, there has been no solid state ballast circuit which has gained widespread commercial acceptance. There are perhaps many facors which have contributed to the lack of a commercial solid state, high-frequency ballast or inverter for gaseous discharge lamps, among which are the low initial cost of conventional ballasts due to the large volume and efficient production techniques which are employed, thereby at least partially offsetting any reduction in operating costs by reducing the initial outlay. Further, from a technical standpoint, when solid state ballasts were first developed, the power switches that were available for operating at the frequency and current levels required, were either too expensive or not reliable enough. Many of these problems have now been overcome due to advances in technology, and further, the increased cost of energy has emphasized the need for reducing operating costs over the long term.
Manufacturers are, however, still faced with problems. Among such problems is the need to provide protection for the various faults which may occur in a solid state ballast, such as excessive current or voltage in the thyristor power circuit, lamp over-voltage, loss of primary power, lamp failure, and so on.
Considering the various applications, such as fluorescent lamps, low and high pressure sodium, and metal halides, if a separate solid state ballast is required for each such application, and each ballast must include not only the numerous fault protection mechanisms, but also regulate lamp current during normal operation, provide for automatic start up and re-strike during the period between cusps of primary power when oscillation may be extinguished, it can be seen that a solid state ballast capable of widespread application would be extremely desirable. Moreover, the availability of such a ballast would permit those involved in the development of various types of new systems using a particular ballast to shorten development time and lower development costs through the availability of greater knowledge of and experience with the particular ballast.
U.S. patent application Ser. No. 194,783, referenced above, describes a solid state, high-frequency ballast or inverter for energizing a lamp circuit with a high frequency oscillating voltage derived from full-wave rectified line voltage. The ballast includes a thyristor/capacitor inverter bridge the output of which is regulated by a variable delay circuit which is synchronized with the zero crossing of the thyristor current. The solid state ballast includes an initialization circuit for lamp warm-up, a re-strike circuit for re-start when the rectified line voltage falls below that which is necessary to sustain oscillation of the inverter, and fault detection circuits including a thyristor over-current detection circuit.
In the thyristor over-current protection circuit provision is made for sequentially commutating both power thyristors to the off-state when the current through the series-connected thyristors exceeds a predetermined value, as when both thyristors conduct simultaneously (thyristor fault). Thyristor shutdown following the detection of a fault and the removal of the drive pulses from the thyristors is accomplished by gating ON a third thyristor which couples a capacitor having a stored charge between the junction of the thyristors and circuit ground. The charge stored on the capacitor is used to produce a reverse polarity terminal voltage across the poer thyristors which, in turn, sequentially shut off. After the first thyristor shuts off, the natural oscillation of the circuit commutates and the second power thyristor also turns off.
This system requires the use of sophisticated logic and control circuitry for processing the thyristor over-current control signal and initiating the gating ON of the third, or commutating, thyristor. In addition, the charge stored on the capacitor is depleted during one recovery cycle to the extent that if the fault condition persists and as capacitor charge is depleted, recovery becomes impossible and damage to the thyristors may result. This system also uses more circuit components and dissipates more electrical energy then the improved circuit disclosed herein.